1. Technical Field
This invention relates to apparatus for vapor deposition of coatings in general, and to cathodic arc vapor deposition apparatus in particular.
2. Background Information
Vapor deposition as a means for applying a coating to a substrate is a known art that includes processes such as chemical vapor deposition, physical vapor deposition, and cathodic arc vapor deposition. Chemical vapor deposition involves introducing reactive gaseous elements into a deposition chamber containing one or more substrates to be coated. Physical vapor deposition involves providing a source material and a substrate to be coated in an evacuated deposition chamber. The source material is converted into vapor by an energy input, such as heating by resistive, inductive, or electron beam means.
Cathodic arc vapor deposition involves a source material and a substrate to be coated placed in an evacuated deposition chamber. The chamber contains only a relatively small amount of gas. The negative lead of a direct current (DC) power supply is attached to the source material (hereinafter referred to as the “cathode”) and the positive lead is attached to an anodic member. An arc-initiating trigger, at or near the same electrical potential as the anode, contacts the cathode and subsequently moves away from the cathode. When the trigger is still in close proximity to the cathode, the difference in electrical potential between the trigger and the cathode causes an arc of electricity to extend therebetween. As the trigger moves further away, the arc jumps between the cathode and the anodic chamber. The exact point, or points, where an arc touches the surface of the cathode is referred to as an arc spot. Absent a steering mechanism, an arc spot will move randomly about the surface of the cathode.
The energy deposited by the arc at an arc spot is intense; on the order of 105 to 107 amperes per square centimeter with a duration of a few to several microseconds. The intensity of the energy raises the local temperature of the arc spot to approximately equal that of the boiling point of the cathode material (at the evacuated chamber pressure). As a result, cathode material at the arc spot vaporizes into a plasma containing atoms, molecules, ions, electrons, and particles. Positively charged ions liberated from the cathode are attracted toward any object within the deposition chamber having a negative electrical potential relative to the positively charged ion. Some deposition processes maintain the substrate to be coated at the same electrical potential as the anode. Other processes use a biasing source to lower the potential of the substrate and thereby make the substrate relatively more attractive to the positively charged ions. In either case, the substrate becomes coated with the vaporized material liberated from the cathode.
Cathodic arc deposition has historically been used to apply relatively thin coatings (2-5 mils; 1 mil=25.4 microns=25.4×10−6 m) to substrates. The deposition rate of cathodic arc coaters is typically relatively slow (e.g., 0.3 mil/hr), but has not been a substantial concern in view of the thin coatings. Applying a relatively thick coating (10-200 mils) has heretofore not been practical because of the substantial time required to apply the coating at the deposition rate of currently available cathodic arc coaters (e.g., at 0.3 mil/hr, a 150 mil thick coating would take at least 500 hrs)
It may be possible to slightly increase the deposition rate of an existing cathodic arc coater by only increasing the current applied. The increase in deposition rate is limited however, by the apparatus currently available. Significantly increasing the current in currently available cathodic arc coaters is likely to cause the portion of the cathode proximate the arc to undesirably melt or increase the chance that undesirable macroscopic particles will be liberated. In addition, most currently available cathodic arc coaters could not accommodate an appreciable increase in current without incurring significant damage.
In short, what is needed is an apparatus and method for cathodic arc vapor deposition of material on a substrate that is capable of operating at a high deposition rate.